In recent years, electric cars using an electric motor as a drive source and hybrid cars using an electric motor and internal combustion engine as drive sources are attracting attention. These vehicles are normally provided with a chargeable battery and the electric motor is driven by electric energy stored in this battery.
The battery includes internal resistance, and therefore charging/discharging is accompanied by heat generation. For this reason, the temperature of the battery rises as charging/discharging of the battery is repeated.
Furthermore, when such a battery is continuously used under a high-temperature condition, the life is generally shortened. For this reason, to protect the battery, the battery may be controlled so as to limit the charge/discharge current when the battery is under a high-temperature condition. However, in this case, sufficient propulsion may not be obtained or in the case of a hybrid car, poor fuel efficiency of the internal combustion engine may result.
For this reason, the above described vehicle is often provided with a fan for cooling the battery. The rotation speed of the fan is normally high when the battery is under a high temperature condition and slow under a low temperature condition.
On the other hand, in the case of the hybrid car or the electric car, when the car is parked or when the electric motor is used as a drive source, generally noise in the passenger compartment is often kept to a low level. When noise is at a low level in such a passenger compartment, if the fan is operating for cooling the battery, the operating sound of the fan increases relative to background noise in the passenger compartment, that is, noise other than the operating sound of the fan. Since passengers of the vehicle are rarely aware of the reason for the operation of the fan under ordinary circumstances, the passengers feel the operating sound of the fan as offensive to the ear.
Therefore, in order to reduce annoyance by the operating sound of the fan, the fan is stopped when, for example, the vehicle is parked and no operating sound of the internal combustion engine is present. Alternatively, measures such as reducing the rotation speed of the fan are taken. For example, Japanese Laid-Open Patent Publication No. 2001-103612 proposes a fan control apparatus for a hybrid car provided with a function for reducing the rotation speed of the fan before stopping idle operation when stoppage of the idle operation of the internal combustion engine is predicted. As the fan control apparatus disclosed in this publication, the rotation speed of the fan decreases prior to the stoppage of idle operation, and therefore it is possible to reduce annoyance of the passengers caused by the operating sound of the fan while the idle operation is stopped.
However, it is not always possible to determine whether background noise in the passenger compartment, that is, noise other than the operating sound of the fan is relatively small or not based only on whether the idle operation of the internal combustion engine is stopped or not. That is, even if the idle operation is stopped, background noise may be large in the passenger compartment due to sound other than the operating sound of the internal combustion engine. When the rotation speed of the fan continues to be low for a long time though background noise in the passenger compartment is relatively large, the temperature of the battery increases and the life of the battery may be shortened.
In the case of not only a battery for supplying electric energy to a vehicle motor, but also other vehicle-mounted batteries which realize cooling through rotation of the fan, the above described situation in which the operating sound becomes offensive to the ear is generally common.